The present invention relates to a new composition which can be used as a separating agent for the manufacture of products in the form of slabs consisting of a granulate or particles of stone material bonded with a hardening resin.
The method for the manufacture of slabs to which the present invention relates is one in which a mixture consisting of a granulate of selected particle size and a synthetic resin is deposited in metered quantities on a conveyor belt which is made to advance to a moulding station where it undergoes a vacuum compacting action with simultaneous application of a vibrating movement of predefined frequency and then to a station for hardening of the resin (preferably by means of the action of a catalyst and/or heat).
Prior to the moulding station, the upper surface of the metered quantity of mixture deposited on the conveyor belt is covered with a sheet or layer of protective material which prevents the pressing ram of the press from soiling the mixture.
In the past this protective sheet material was paper (IT-A-1,056,388 and IT-A1,117,346), use of which, however, was accompanied by a few secondary problems downstream of the actual production line. After hardening of the resin, the sheet of paper in fact adhered firmly to the surface of the hardened slab and removal from the slab surface proved to be very difficult; for this reason, during finishing of the slab, it was necessary to perform a special operation for mechanical removal of the protective paper.
Subsequently an improvement was introduced (described and claimed in EP-A-0.786.325) whereby the layer or sheet of paper is replaced by a sheet of elastic material, preferably rubber, such that, at the end of the stage involving hardening of the bonding resin, the rubber sheet may be removed by means of tearing from the surface of the finished slab and subsequently reused.
In a preferred embodiment of the abovementioned improvement, the support, on which the mixture is deposited before being transferred to the vacuum and vibration compacting station, is also protected with a similar sheet made of elastic material, in particular rubber.
The mixture deposited on the support and enclosed between the two rubber sheets after the moulding stage is in the form of a slab, at the edges of which the two protective rubber sheets are joined to one another with their respective edges superimposed, enclosing in a substantially complete manner the rough-formed slab which has not yet undergone the stage of hardening of the resinous binder. Since, during compaction, an albeit minimal part of mixture inevitably forms a burr located between the two edges, after hardening of the resinous binder this burr forms on the rubber sheets incrustations which are difficult to remove, due to the fact that, during hardening, the two rubber edges do not remain in contact with each other, resulting in separation of the not yet hardened burr from the body of the product.
In order to overcome this problem, still according to the previously known method, a separating liquid is applied to the two edges to be joined together, immediately prior to use along the production line.
Despite these measures it is still necessary to perform operations involving cleaning of the two rubber sheets, in particular along the two mating edges, said operations being certainly less difficult than the prior operations associated with removal of the protective paper, but nevertheless requiring an additional operation in the production cycle.
WO 99/21695 proposes an alternative solution which does not require the use of the separating agent: the flat configuration of the bottom sheet is in fact replaced by a shaped configuration, comprising a flat bottom and a perimetral border projecting from said flat bottom over a predetermined height, so as to define a seat with dimensions corresponding in plan view to those of the slab to be obtained, but the aforementioned perimetral border is provided with a height which is smaller, by a predetermined value, than that of the final slab which is to be obtained so that, after the compacting stage, a space of predetermined thickness remains between the perimetral edge of the upper sheet and the top of the aforementioned border.
Surplus mixture material fills this space of predetermined thickness during the vibrating compression under vacuum, said material subsequently remaining attached to the edge of the slab during the hardening stage.
When, after the stage involving hardening of the resinous binder, the final slab is released, by means of tearing, from the two rubber sheets, a rough hardened slab is obtained, said slab having perimetrally a border of hardened material which may be easily removed during the usual finishing operations carried out on a slab of stone material.
However, both in the method described in EP-A-0.786.325 and in the method described in WO 99/21695, a further problem consists in the deterioration of the rubber sheets caused by the styrene present in the resinsxe2x80x94something which obviously reduces significantly the working life of the said rubber sheets.